Tracking error signals are used in optical storage systems to control the position of an incident read and/or write radiation beam with respect to a data track on an optical storage medium. Most systems use either a phase tracking or a density tracking technique to detect the positional error between the incident radiation beam and the data track.
Phase tracking typically involves detecting variations in diffraction patterns resulting from interaction between the incident beam and the medium. The diffraction pattern variations may result from movement of the incident beam relative to a pre-formed diffracting structure, often referred to as a groove, pregroove or guide track, on the medium surface. Other phase tracking techniques use diffraction pattern variations resulting from movement of the incident beam relative to preformatted tracking marks, also referred to as servo marks, placed on or near the data tracks of, for example, a phase-change or ablative medium. Exemplary phase tracking techniques, including those commonly referred to as push-pull tracking and sampled phase tracking, are described in pp. 172-178 and 180-181 of A. Marchant, "Optical Recording: A Technical Overview," Addison-Wesley, Reading, Mass., which are incorporated by reference herein. Known phase tracking techniques, however, typically suffer from a number of disadvantages. For example, phase tracking systems may require additional optics and/or detectors in the return path, and nay be sensitive to phantom tracking, which refers to false error signals resulting from changes in the relative positions of the incident beam and an objective lens used to focus the beam on the medium.
In density tracking, a density signal is detected which typically corresponds to an amount of incident radiation falling on and reflected by a given data mark or tracking mark. Exemplary density tracking techniques include multi-spot tracking, wobble tracking, or sampled density tracking. Both multi-spot tracking and wobble tracking may also be implemented as phase tracking techniques, by detecting variations in diffraction patterns produced by the wobbled or multi-spot incident beams, respectively, rather than a density signal.
Multi-spot tracking, also referred to as outrigger tracking, generally uses a single optical source and multiple illumination spots to generate a tracking error signal. An exemplary phase-type multi-spot tracking system is described in U.S. Pat. No. 4,787,076 and uses a diffraction grating in the incident beam path to produce three spots, corresponding to diffraction orders 0, +1 and -1, from a single beam. The three spots include a main beam spot for reading and/or writing data on a medium, and two auxiliary beam spots, or outrigger spots, offset in both in-track and cross-track directions from the main beam spot. The diffraction patterns resulting from application of the two auxiliary beam spots to the edges of a pregroove on a magneto-optic medium are detected and processed to provide a phase-type tracking error signal. Such prior art multi-spot tracking systems, however, suffer from a number of drawbacks. For example, the use of a grating to generate multiple spots substantially reduces optical head efficiency and in some cases may require precise alignment of optical components. In addition, separate detectors are usually required for each of the three spots, resulting in a higher component count and increased optical head cost and complexity. Furthermore, both phase-type and density-type multi-spot tracking techniques may produce a tracking error signal which is overly sensitive to the presence of neighboring tracks. Further detail regarding outrigger tracking may be found in, for example, pp. 178-180 of the above-cited A. Marchant reference, which are incorporated by reference herein.
On a magneto-optic storage medium, data is generally stored in the form of marks having a distinct magnetization, such that an incident radiation beam reflected from the medium can detect the marks and thereby the recorded data. Most commercially-available magneto-optic storage media include a pregroove which, as noted above, is suitable for generating a phase tracking error signal. Without either a pregroove or a previously-recorded data track from which variations in diffraction or reflection can be measured in, for example, a multi-spot system, magneto-optic media are generally unable to generate a suitable phase or density tracking error signal. Furthermore, it is generally difficult to optimize a magneto-optic system to simultaneously provide both a phase tracking error signal and a read-out data signal of sufficiently high quality.
Other types of optical media can generate phase tracking error signals without the need for a pregroove or a recorded data track. For example, a number of known write-once optical media utilize the above-mentioned preformatted tracking marks to generate a sampled phase tracking error signal. As used herein, write-once media include read-only media, such as compact disks (CDs), which are often reproduced in mass quantities from a master recording. In general, however, tracking marks have not been used to provide tracking error signals for magneto-optic media. The tracking signal schemes used for non-grooved magneto-optic media are thus often incompatible with those used for other types of non-grooved media, such as non-grooved write-once media. As a result, many optical storage media drives may be unable to handle both magneto-optic and write-once optical media. Multi-function optical drives may not be practical unless techniques are developed which allow, for example, magneto-optic media to operate using tracking error signals similar to those used for write-once media.
As is apparent from the above, a need exists for improved density-type tracking error signal generation in multi-spot optical systems such that accurate tracking is provided on a variety of different types of media, including both magneto-optic and write-once media, without requiring separate tracking detectors, a media pregroove or a previously-recorded data track.